The present invention relates to a booster enclosure comprising a pair of shells which are mutually coupled together and, more particularly, to an improvement in a coupling structure of such a pair of shells.
A booster such as a brake booster or a clutch booster incorporates a housing comprising a pair of shells which are coupled to each other. A conventional coupling structure of this type is known in which a front shell is cup-shaped overall and a rear shell is plate-shaped; the rear shell fits inside the front shell; engaging pawls of the front shell prevent the rear shell from dropping off; and an annular groove which opens radially outwardly on a portion of the outer circumference of the rear shell and a matching portion of the inner circumference of the front shell clamp an outer circumferential beaded part of a diaphragm. Since the diameter of the coupling parts of the front shell and the rear shell need not be made larger than that of the other parts with such a coupling structure, the diameter of the coupling structure may advantageously be reduced. However, the outer circumferential rim of the rear shell contiguous with the annular groove of the rear shell and in contact with the engaging pawls of the front shell has conventionally been a flat annular rim extending in the radial direction. For this reason, when the thickness of the shell is reduced to reduce the overall weight of the booster, this rim is bent inward by the force acting on it when the brake force is exerted, resulting in incomplete coupling of the two shells and incomplete sealing of the diaphragm.
In order to solve this problem, a reinforcing plate may be spot-welded to the above-mentioned outer circumferential rim of the rear shell. However, the attachment of the reinforcing plate by welding counteracts the weight-reducing effect obtained by reduction of the thickness of the shell and also adds an extra manufacturing step.